[subject] + 不失为 + 一 + [measure word] + [adjective] + [noun]
不失为 says something can fairly be called a good option, even if it isn't the best or the obvious one. It goes before a noun phrase, usually 一个 + adjective + noun. The register is written or careful speech, and 为 is read wéi.
The word carries a concession: the speaker grants the point, often after a doubt or a drawback has come up.
是 states the judgement outright. 不失为 grants it with some reserve – "fair enough, it counts".
不失为 needs a noun phrase after it. An adjective on its own, or a negative judgement, doesn't fit.